In a double reclining mechanism provided with reclining mechanisms each tilting a seat back with respect to a seat cushion on both sides of a seat, a connection rod provided with splines or serrations is used to transmit an action of one reclining mechanism to the other reclining mechanism (see, for example, JP-A-2002-101996).
The connecting rod described in JP-A-2002-101996 is manufactured by providing a long rod material with splines on an entire peripheral surface by extrusion molding followed by cutting in a predetermined length by shearing work